1. Field of the Invention
This invention relates to a pressure measurement sonde utilizable in particular for carrying out pressure measurements in underground wells, such as oil wells. More particularly, the invention relates to improvements made in a pressure measurement sonde in order to correct the effects due to temperature as well a to temperature variations. The improvements also concern the mechanical design of the sonde.
2. Prior Art
It is current practice in the petroleum industry to carry out pressure measurements in wells producing hydrocarbons. Pressure is in fact an important physical characteristic for the production and operation of oil wells. Thus, during tests conducted before a well goes into production, a pressure sonde is lowered into the well, a valve is closed in order to stop the flow of fluid and the pressure variations as a function of the time elapsing since the closing of the valve are measured. The curve obtained, called the pressure build-up curve, contributes to the determination of the characteristics of the producing zone. Tests are also conducted on the interference between two neighboring wells, by carrying out pressure measurements in one of the wells as a function of the flow of hydrocarbons in the adjacent well.
The sondes used are lowered into the well at the end of a cable. This cable can be electrical and, in this case, the measurement signals are transmitted directly to the surface. The cable can be nonconducting and, in this case, the information is recorded downhole in a memory contained in the sonde. One of the sondes most widely used at the present time for pressure measurements is certainly the sonde manufactured by the American company Hewlett-Packard under the reference 2813 B. This sonde is described for example in the article entitled "A New, Surface Recording, Down-Hole Pressure Gauge" published by G. B. Miller et al. in the "Society of Petroleum Engineers Journal" under the number SPE 4125, or in the U.S. Pat. No. 3,561,832 (H. E. Karrer et al.).
This sonde includes, in the same enclosure, two crystal detectors, one being subjected to the pressure and temperature conditions prevailing in the well whereas the other is subjected only to temperature. By taking the difference of the indications given by these two crystal detectors, a pressure measurement independent of temperature is obtained. This system however operates prefectly well under static temperature conditions, but the temperature compensation is no longer satisfactory when a rapid variation in the temperature of the surrounding medium occurs. It is then necessary to wait as long as required for the two measurement and reference detectors to reach thermal equilibrium before a proper pressure measurement can be carried out. Furthermore, the mechanical characteristics of the sonde are not satisfactory in every respect.
A solution considered for overcoming these drawbacks consists in determining the pressure variation law measured with the sonde as a function of temperature and in correcting, on the surface, the measurements obtained as a function of said law. This law is not linear and is relatively complex. It is thus difficult to use. Another solution consists in placing the measurement system within an isothermal enclosure, for example a Dewar vessel. However, the sonde must be lowered within tubes of small diameter forming the production string for example. The small diameter of the tubes however makes it difficult to design a sonde with an isothermal enclosure.